1. Field of the Invention
The present invention relates to apparatus for indicating the quantity of fluid contained within a container therefor.
2. Description of the Prior Art
Prior art fluid quantity indicators, such as the type utilized for providing an indication of the quantity of aviation fuel in a given tank, are normally dependent on the particular unique fuel tank configuration in which the indicator is being utilized. These aircraft fuel sensor probes are generally of the capacitive type, such as where the sensing element or probe comprises a concentric tube capacitor, such as disclosed by way of example in U.S. Pat. Nos. 2,866,337 and 3,534,606, preferably mounted in the vertical position in the tank, or in multiples within the tank to average out variations in pitch of the aircraft, and another capacitor, utilized as a reference capacitor, lying in a horizontal position in the bottom of the tank. A capacitance change, in such prior art fluid quantity indicators, occurs proportional to the amount of fluid or fuel in the tank since the dielectric constant of aviation fuel, by way of example, is different from that of air. This reference capacitor probe located in the bottom of the tank, normally remains submerged at all times and compensates for variation of the dielectric constant of the fuel with changes in temperature, pressure, and fuel type. In such prior art fuel quantity indicators, the sensing elements and the reference capacitors in the tank are normally not held to a precise zero fuel and full tank capacitance and, accordingly, since these values change from aircraft to aircraft for the same tank; a zeroing adjustment of the fuel quantity indicator is required at each installation of an indicator. In addition, a different full scale factor must be set for each type or capacity tank. Accordingly, depending upon the failure rate of the fluid quantity indicator, a substantial amount of time in rezeroing the indicators and in resetting the scale factor is expended since the indicator has to be zeroed upon replacing a failed unit and the scale factor has to be set for each different capacity fuel tank. On an average, there are three to four fuel tanks per aircraft with a varying fuel capacity and variable changes in capacitance per unit volume so that in a given aircraft fuel tank, such as one having a 10,000 pound fuel capacity, the tank might have a capacitance change of the sensor from empty to full of 1,000 picofarads and another fuel tank on the same aircraft might have a fuel capacity of 80,000 pounds with an identical probe capacitance change. This lack of standardization of capacitance change versus fuel quantity requires a separate indicator for each tank with its full scale indication adjusted for the appropriate capacitance change of that fuel tank. Accordingly, this lack of standardization in prior art fuel quantity indicators which make them fuel tank dependent requires the maintenance of a large inventory of such indicators for use in several different types of aircraft with different capacity fuel tanks. Furthermore, prior art fuel quantity indicators have generally previously been primarily analog systems which, in the field of airborne instrumentation, primarily utilize moving parts which are normally less reliable than systems without such moving parts, particularly when subjected to extreme environmental conditions and have not been satisfactorily lightweight and compact. In an attempt to overcome some of these disadvantages, some recent prior art fluid quantity indicators have been digital systems, such as the type disclosed in U.S. Pat. Nos. 3,301,056 and 3,463,980, as opposed to analog systems, such as the analog servo bridge type of system, such as disclosed in U.S. Pat. No. 3,079,797. These digital prior art systems, however, suffer from the same disadvantage of the prior art analog systems with respect to being dependent on the particular fuel tank configuration so that the scale factor and the zero set for a given fuel tank has to be accomplished each time a new indicator is installed and/or is utilized in a different capacity fuel tank. These disadvantages of the prior art are overcome by the present invention.